Tachyarrhythmias are abnormal heart rhythms characterized by a rapid heart rate, typically expressed in units of beats per minute (bpm). They can occur in either chamber of the heart (i.e., ventricles or atria) or both. Examples of tachyarrhythmias include ventricular tachycardia, ventricular fibrillation, atrial tachycardia, and atrial fibrillation. Tachycardia is characterized by a rapid rate, either due to an ectopic excitatory focus or abnormal excitation by normal pacemaker tissue. Fibrillation occurs when the chamber depolarizes in a chaotic fashion with abnormal depolarization waveforms as reflected by an EKG.
An electrical shock applied to a heart chamber (i.e., defibrillation or cardioversion) can be used to terminate most tachyarrhythmias by depolarizing excitable myocardium, which thereby prolongs refractoriness, interrupts reentrant circuits, and discharges excitatory foci. Cardioversion refers to a shock delivered synchronously with a ventricular beat, while defibrillation refers to an asynchronously delivered shock. Implantable cardioverter/defibrillators (ICDs) provide this kind of therapy by delivering a shock pulse to the heart when fibrillation is detected by the device. An ICD is an electronic device containing a pulse generator that is usually implanted into the chest or abdominal wall. Electrodes connected by leads to the ICD are placed on the heart, or passed transvenously into the heart, to sense cardiac activity and to conduct the shock pulses from the pulse generator. ICDs can be designed to treat either atrial or ventricular tachyarrhythmias, or both, and may also incorporate cardiac pacing functionality.
The most dangerous tachyarrhythmias are ventricular tachycardia and ventricular fibrillation, and ICDs have most commonly been applied in the treatment of those conditions. Ventricular fibrillation is treated by the immediate delivery of a defibrillation shock, while ventricular tachycardia can be treated with either a defibrillation or a cardioversion shock. ICDs are also capable, however, of detecting atrial tachyarrhythmias, such as atrial fibrillation and atrial flutter, and delivering a shock pulse to the atria in order to terminate the arrhythmia. Although not immediately life-threatening, it is important to treat atrial fibrillation for several reasons. First, atrial fibrillation is associated with a loss of atrio-ventricular synchrony which can be hemodynamically compromising and cause such symptoms as dyspnea, fatigue, vertigo, and angina. Atrial fibrillation can also predispose to strokes resulting from emboli forming in the left atrium. Although drug therapy and/or in-hospital cardioversion are acceptable treatment modalities for atrial fibrillation, ICDs configured to treat atrial fibrillation offer a number of advantages to certain patients, including convenience and greater efficacy.
As aforesaid, an ICD terminates atrial fibrillation by delivering a shock pulse to electrodes disposed in or near the atria. The resulting depolarization also spreads to the ventricles, however, and there is a risk that such an atrial shock pulse can actually induce ventricular fibrillation, a condition much worse than atrial fibrillation. To lessen this risk, the ICD delivers a cardioversion shock synchronously with a sensed ventricular depolarization (i.e., an R wave) and delays delivering the shock until the intrinsic ventricular rhythm is below a specified maximum rate. That is, an R—R interval, which is the time between a presently sensed R wave and the preceding R wave, is measured. If the R—R interval is above a specified minimum value, the interval is considered shockable and the atrial cardioversion shock pulse is delivered.
If the ventricular rhythm does not slow to a safe rate in a short time, however, the delay in delivering atrial cardioversion therapy may be deleterious to the patient. In addition, a particular subset of patients have spontaneous ventricular rhythms during atrial tachyarrhythmias that may remain too rapid to ever safely deliver an atrial cardioversion shock, and these patients may not be suitable candidates for ICD therapy. What is needed in the art is a system which addresses these and other problems.